Open your scanner and lay a small mirror on the bottom of the case. You can now look at the images in the mirror as you position them on the glass.

Haven't tried this yet, but will get around to it one of these days. Should work OK if you make it a small thin one that doesn't interfere with the scanning bar.

If you use HP Precision Scanning S/W, you can also do a prescan to the monitor, make fine adjustments to each chip on the glass, and do the final scan.

DonL